I haven't officially studied maths since I was in secondary school, and wanted to brush up on it, but I'm not sure where to start. I'm a software developer, so my maths skills are not so bad, but I find myself turning to a calculator for many things when I'm sure doing it in my head can't be so hard.

What can you recommend?

I turn to the calculator constantly too!!! I found forcing myself to sit down and do simple problems and progressing to more difficult stuff without a calculator helpful and learning shortcuts/laws/formulas/rules instead of looking them up all the time helps save time and improve skills too.